Devices for preheating of scrap, which is to be charged to a steel furnace, particularly an electrical steel furnace, are known. The preheating of the scrap is carried out in order to partly save energy in the furnace and partly to dry the scrap and remove any ice or other material which may contaminate the steel, which may present pollution problems in the gaseous discharge, or may cause severe accidents when the material is brought in contact with molten metal.
The preheating may be performed by supplying heat from separate burners or by utilizing hot gases, which have been generated during a high temperature process. Particularly, hot gases from the furnace, to which the preheated scrap is to be charged, are utilized in this case. However, the hot gases may also come from another steel furnace or any other furnace, e.g. gas-fired furnaces for annealing and holding. The hot gases may in this case be utilized directly or via an interconnected heat exchange unit.
Such scrap preheating devices have been described in the articles "Scrap preheater for electric arc furnace", Iron and Steel Engineer, April 1983, pages 45-50 (see also UK Patent Publication No. GB 2,082,738 A) and "EF Update 82/Four Scrap preheating systems", 33 Metal Producing, November 1982, pages 50-53. These known devices principally comprise a preheating chamber which is connected to an outlet pipe to remove expended preheating gases. A scrap bucket having a gas-transmitting bottom portion is placed in the preheating device on a suitable support. The preheating chamber and the scrap bucket are covered by a cover having a central inlet connected to an inlet pipe for hot gases. The device is provided with a seal, which is a waterseal, between the cover and the wall of the preheater chamber. Also, there is a seal between the upper edge of the scrap bucket, which comprises an annular seating surface and an annular sealing element on the cover. This sealing element may comprise an edge ring or flange projecting downwardly on an inner hood, which forms a part of the cover, or a part of a separate, e.g. bellows shaped element.
In operation, the hot gases are fed through the inlet pipe and descend through the scrap charge and leave the preheater device through the outlet pipe to be conducted to e.g. a flue gas cleaner before discharge. In this case, a hydraulic head of about 200 mm water column between the top side of the scrap charge and the outside of the scrap bucket will arise.
The artisan has found that it is not possible to obtain an efficient sealing between the seating ring of the scrap bucket and the sealing element of the cover. Due to a normally rough handling, influenced by large forces, which particularly the scrap bucket but also the cover is subjected to during a normal operation, leakage gaps having a size of from 1 to 5 mm between the seating ring and the sealing element will soon appear. Consequently, a substantial portion of the hot gases will issue through these leakage gaps without passing through the scrap. In this manner, efficiency of preheating in relation to the heat quantity available in the preheating gases is reduced. This results in large economical losses, since that energy, which principally may be obtained free of charge from the preheating gases, must be replaced by expensive electrical energy in the electrical steel furnace to heat the scrap before the smelting. These economical losses are not limited to just the direct energy expenses. Thus, when an increased preheating is used, a shorter charging time can be obtained, which results in an increased utilization of the entire steel mill, and thus reduced capital outlay for each ton of produced steel. Each minute, with which the smelting time can be reduced due to an increased preheating, is extremely valuable.
The artisan has already tried to stop leakage between the seating ring of the scrap bucket and the sealing element of the cover. However, considerable inconveniences are then encountered due to the high temperature of the preheating gases, often about 800.degree.. He has tried to use sand as a sealing material but only with limited results. He has also tried a sand-containing asbestos hose. However, the asbestos hose has a very limited durability under actual operating conditions and may contribute to an environmental danger. Thus, none of the previously suggested solutions results in a satisfactory solution to the problem due to leaking hot gases.